1. Field of the Invention
This invention relates to processes and apparatus for treating sewage sludge or similar organic sludge, such as that obtained from industrial sites such as waste water treatment plants.
2. Description of the Prior Art
EP 0 283 153 B1 discloses a method of treating wastewater sludge to provide a fertiliser for agricultural lands which can be applied directly to the land. The method involves mixing the sludge with alkaline material of specified fineness to raise the pH of the mixture to at least 12 for at least a day and to effect pasteurisation, and then drying the mixture. Drying is done either a) by aeration and maintaining the pH above 12 for at least seven days until the solids levels reach and maintain a minimum of 65% bulk solids, or b) by aeration and heating to a temperature of at least 50xc2x0 C. and so that the solids levels reach and maintain a minimum of 50% solids. In b) the temperature should not be so high as to destroy all non-pathogenic organisms. The elevation of the pH and the drying are effected so as to reduce odour and undesirable viruses, bacteria, parasites and vector (e.g. flies) attraction to the sludge and prevent significant pathogen regrowth while not eliminating beneficial non-pathogenic microorganisms. At least some of the temperature rise is due to the exothermic reaction with the alkali. The product is allowed to air cure for about 10 days after achieving the desired solids content. Drying and curing may be effected by windrowing, turn-over or other forced air methods, and curing or aeration time is dependent on the aeration procedure and other factors, such as ambient temperature and humidity, and clearly need to be determined, for any particular circumstances, experimentally to see that the required end point has been reached. This involves taking samples to measure the solids content, testing for:
animal virusesxe2x80x94less than one plaque forming unit per 100 ml
salmonella bacteriaxe2x80x94less than three colony forming units per 100 ml
parasitesxe2x80x94less than one viable egg per 100 ml
significant regrowth of the pathogenic microorganismsxe2x80x94there should be none
and assessing the reduction of odour to a level tolerable in a closed room and that this reduction is maintained indefinitely under any climatic conditions, and whether flies are less attracted to the product. At the same time, the presence of at least some beneficial non-pathogenic microorganisms must be established.
If method b) is used, the minimum recommended time for maintaining the temperature at least 50xc2x0 C. (but not so high as to destroy all non-pathogenic microorganisms) is at least 12 hours and the heat treated alkaline stabilised dewatered sludge cake is then air dried (while the pH remains above 12 for at least 3 days) through intermittent turning of windrows until a minimum of 50% solids content is achieved.
GB 2 276 876 A, which refers to EP 0 283 153, describes treatment of sewage sludge having a solids content of at least 15% by weight with an alkaline material containing free lime, and storing and/or drying and/or composting the resultant mixture, adding sufficient lime to the mixture to achieve a pH of at least 10. Higher pH values, even higher than 12, are not ruled out, but it is suggested that excellent reduction in pathogens can be achieved at pH levels below 12 with reduced evolution of ammonia, which renders the treated sludge and its surroundings less unpleasant and means that the treated material retains more nitrogen increasing its value as a fertiliser.
The method of GB 2 276 876 involves dewatering the sludge, e.g. in a belt press, and measuring it using a weigh hopper into a mixer into which is delivered alkaline material in the appropriate amount, the mixed material being delivered to a skip to deliver it to a windrowing area where it is turned periodically, normally for a period up to seven days. However, it is suggested also that provided the amount of calcium oxide that has been introduced is sufficient to raise the pH to a value greater than 12, and sufficient, indeed, to ensure that it stays above 12 for at least 2 hours, then it is possible to store the mix for not less than two hours, but then apply it directly to the land.
It is apparent that the methods described in these two specifications are labour intensive and to a large extent dependent upon a subjective judgement (the assessment of odour in a closed room, for example) or upon results of biological tests which themselves could take days or weeks before you have a result.
It is necessary, for effective treatment, to mix the alkali with dewatered sludge, but sludge, nonetheless that comprises a substantial quantity of water, as it is only in the presence of water that the desired exothermic chemical reaction takes place. The end productxe2x80x94as is clear from the specification discussedxe2x80x94is required to have less water content than is appropriate during the heat-development stage. It is an essential part of the process, then, that the product be eventually dried, and this is the reason for treating it by windrowing. Windrowing, especially aeration by turning over windrows, is not only labour intensive and floor space intensive, but is a slow method of drying, and exposes personnel to excessive levels of ammonia and volatile organic compounds.
There are further problems and disadvantages with prior art methods such as those disclosed in EP 0 283 153 and GB 2 276 876. For example, a relatively large quantity of alkaline material is required (typically 30 to 50% by sludge wet weight or 150-200% by sludge dry weight): so much so that the alkaline material constitutes a significant proportion of the end product produced by the treatment. This adds to costs in three significant ways. Firstly, there are costs associated with providing large quantities of alkaline material. Secondly, there are considerable costs associated with materials, handling of increased mixed product volume through the heat pulse and windrow formation and turning. Thirdly, there are transportation costs associated with the removal of the end product from the site. A further consequence is that, because a significant proportion of the end product is alkaline material, the relative proportions of nutrients such as nitrogen and phosphorus in the end product are not only reduced but also are fixed due to the high pH of the product. This reduces the possible value of the end product as an agricultural fertiliser. Furthermore, since such large quantities of alkaline materials are added, a pH greater than 12 is maintained for extended periods and may result in the product being classified as a special waste by regulatory bodies. Special waste requires special land fill disposal.
It is difficult, if not impossible, to evenly control, culture and measure the temperature of a windrowed product. Furthermore, it has been found that, in practise, considerable problems with product odour, in particular ammonia, can be encountered when the method of EP 0 283 153 is employed, both during production and in the finished product.
It should be noted that, under United States of America regulations for the treatment of sewage sludge (EPA Rule 503 Regulations), it is necessary not only to effect pasteurisation (Class A pathogen reduction) but also to effect stabilisation of the product. The latter is termed the Vector Attraction Reduction Requirement, and is described at Rule 503.33(b). For the avoidance of doubt, the terms xe2x80x9cstabilisersxe2x80x9d and xe2x80x9cstabilisationxe2x80x9d as used herein refer to a process which satisfies the Vector Reduction Requirement as set forth in US EPA Rule 503.33(b).
The present invention overcomes the aforesaid problems and disadvantages and provides a new paradigm in the treatment of sewage sludge. The present invention provides processes and apparatus for treating sewage sludge which are more efficient, less labour and space intensive, and more controllable, (so as not to depend upon the results of bio-technological testing, inaccurate manual sampling, and measurement procedures such as dry solids content, temperature and pH) than prior art processes to date, and which provide both continuous processing and on line process monitoring and management. This ensures that the critical process steps which ensure pathogen destruction and subsequent long term stability of the treated sludge are achieved. In particular, the tonnage of end product produced after the treatment of a given amount of sludge is significantly reduced, resulting in lower production, handling and transportation costs. This is principally because the amount of sludge and alkaline material required in order to effect pasteurisation is significantly reduced. A further advantage associated with reduced usage of alkaline material is further cost savings. Furthermore, in contrast to the methods of EP 0 283 153 and GB 2 276 876, the present invention does not involve the use of aeration methods such as windrowing. Furtherstill, the floor space (or xe2x80x9cfootprintxe2x80x9d) required by the apparatus of the present invention is significantly reduced. Yet further, odours produced by the present invention are minimised and easily dealt with. Yet further still, the end product produced by the process of the present invention is of superior quality being high in available nutrients and substantially odour free. Furthermore, since significantly less alkaline material is utilised, both process temperature control and post process pH reduction are achieved more effectively.
According to a first aspect of the present invention there is provided a process for treating sewage or similar organic sludge to produce a pasteurised and dried product comprising the steps of:
drying the sludge to substantially the desired dry solids content of the product;
and thereafter adding an alkaline admixture to the dried sludge in order to pasteurise and/or stabilise the sludge and thereby produce the product without employing subsequent drying by aeration.
There are numerous advantages which accrue from the approach of drying the sludge prior to, or consequent with, pasteurisation (as opposed to drying after pasteurisation, as disclosed in EP 0283153 and GB 2276876). Post drying, the sludge weight and volume is significantly reduced when comparing feedstock output quantities. Only a very small total amount of alkaline admixture is required to pasteurise (and/or stabilise) the mixture. These provide significant cost advantages and reduce the tonnage of product associated with a given amount of wet sludge starting material. Despite the low quantities of alkaline admixture used, excellent temperature pasteurisation and stabilisation profiles over an extended period of time can be obtained. Furthermore, end products having excellent, higher dry solids contents can be readily produced. It is well known that the higher the product dry solids content, the better the long term stability of the end product. Further time and space intensive and costly drying steps, such as windrowing or other aeration techniques are not required. The method can be used to treat raw sludge feedstock, which has the advantage that an expensive sludge digester stage is not required. However, the method can also be used to treat digested sludge feedstock.
The alkaline admixture may be added to the dried sludge at a mixing ratio of less than 25% by weight of sludge dry solids, preferably less than 20%, most preferably less than 15%. This represents a very low proportion of the eventually formed product, but is, surprisingly, sufficient to pasteurise and/or stabilise the sludge and produce high quality product.
Preferably, the alkaline admixture comprises lime which, advantageously, may be pulverised or otherwise finely granularised. It has been found that the greater surface area associated with lime treated in this manner improves the pasteurisation and stabilisation processes. The drying of the sludge may be effected by a thin film dryer, which may be a horizontal, single pass, indirectly heated thin film dryer. Such dryers are manufactured by Buss-SMS GmbH of Pratteln, Switzerland. These dryers operate on a plug flow basis, i.e., first in first out (FIFO), which has the advantage that the residence time of sludge in the dryer can be accurately established. Also, it is possible to completely pasteurise the sludge during the drying step.
The sludge may be dried to a dry solids content of greater than 50%, preferably in the range 52 to 65%. An advantage of limiting the drying to this range is that high fibre content sludges (with consequent risks of fires and explosions in the dried sludge) are not produced.
The method may further comprise the step of dewatering the sludge to produce dewatered sludge cake prior to further drying of the sludge.
The step of drying the sludge may effect partial or complete pasteurisation of the sludge. This assists in keeping the amount of alkaline admixture required to a minimum. Additionally, it should be noted that since the drying of the sludge is not accomplished at high pHs, the odours associated with this drying step do not include ammonia. Rather, volatile species or gases such as volatile organic compounds, hydrogen sulphide and various mercaptans are produced, which can be easily removed by a biological filter, a scrubber or a regenerative thermal oxidiser. The effect of this is to significantly reduce the odour associated with sludge treatment (ammonia and certain volatile organic compounds being the prime sources of concern regarding such odours). If an indirect design of dryer is utilised, only the non-condensible gas stream needs to be scrubbed, and thus the volume of gas, and consequently the scrubber size, is minimised.
The product may be continuously discharged. The pasteurisation and/or stabilisation of the mixture may be performed in a FIFO (first in first out) hopper with or without recycling of the mixture.
Green waste, such as grass clippings and leaves, may be added to the sludge prior to the drying of the sludge to substantially the desired dry solids content.
Gases may be removed during pasteurisation and introduced to a dryer which is used for drying the sludge. An advantage is that particulates, ammonia and volatile organic compounds (VOCs) emanating from the pasteurisation process are removed at the dryer. Particulates can cause problems if introduced to a waste treatment stage such as a regenerative thermal oxidiser (RTO), but if waste streams are introduced to the RTO from the dryer (some having been routed from a pasteurisation unit), then these problems are avoided. A further advantage is that the size of the RTO is minimised, since the gases for the pasteurisation stage substitute for leakage air which is necessary for efficient operation of the dryer.
Surplus heat associated with the drying of the sludge may be recovered, and used for a purpose associated with the process or a facility performing the process. The recovered heat may be used for production space heating and/or to heat an ammonia scrubbing process. In this way, really efficient use of energy is achieved.
The method may be continuous, although batchwise or semicontinuous operation is possible.
According to a second aspect of the invention there is provided a facility for treating sewage sludge or similar organic sludge according to the first aspect of the invention; comprising:
means for drying the sludge to substantially the dry solids content of the product;
means for adding an alkaline admixture to the dried sludge; and
means for retaining the mixture and the added alkaline admixture in order to pasteurise and/or commence stabilisation of the mixture and thereby produce the product without employing subsequent drying by aeration.
The means for retaining the dried sludge and the added alkaline admixture may comprise a first in first out (FIFO) hopper.
The facility may further comprise means for dewatering the sludge prior to further drying of the sludge, which means may comprise a belt press arrangement, which might be a multi-stage belt press.
The means for drying the sludge may comprise a thin film dryer, which may comprise a horizontal, single pass, indirectly heated thin film dryer.
The facility may further comprise at least one store containing lime, the lime being used as the alkaline admixture or as part of the admixture. The lime may be pulverised or otherwise finely granularised.
The facility may further comprise conducting means to remove gases from the means for retaining the mixture and to introduce said gases to the means for drying the sludge. This can replace leakage intake air which is traditionally required with dryers.
The facility may further comprise heat recovery means for recovering surplus heat associated with the drying of the sludge and conducting the recovered surplus heat to a destination so that the recovered surplus heat can be used for a purpose associated with the process or the facility. The facility may further comprise an ammonia scrubbing system which can be heated using the recovered surplus heat.
The means for drying may be adapted to also effect complete pasteurisation of the sludge.
According to a third aspect of the invention there is provided a dried sludge product produced by the process of the first aspect of the invention. Such product can have a high dry solids content (50% or greater), and is high in nutrients. The product can be used as a manufactured top soil product, and as agricultural fertiliser.